Multilayered air-fuel ratio sensor

ABSTRACT

A multilayered air-fuel ratio sensor consists of a plurality of substrate layers. At least one heterogeneous boundary layer is interposed between the plurality of substrate layers. The heterogeneous boundary layer has a thickness in a range of 10 to 100 μm. The heterogeneous boundary layer absorbs thermal shocks or any other stresses acting on the substrate layers and stops the growth of cracks.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a multilayered air-fuel ratiosensor used for controlling an air-fuel ratio of a gas mixture suppliedto a combustion chamber of an internal combustion engine.

[0002] To suppress energy loss (i.e., fuel loss) and prevent serious airpollution, using an air-fuel ratio sensor is inevitably required forpresent passenger vehicles.

[0003]FIGS. 13 and 14 show a conventional multilayered air-fuel ratiosensor disclosed in Japanese Patent No. 2-62955 corresponding to U.S.Pat. No. 5,288,389.

[0004] As shown in FIG. 13, a multilayered air-fuel ratio sensor 9comprises multiple layers consisting of a solid electrolytic substratelayer 91, an insulating spacer 92, a solid electrolytic substrate layer93, and a shielding plate 94.

[0005] As shown in FIG. 14, the multilayered air-fuel ratio sensor 9comprises a pump cell 919 and a sensor cell 939. A sample gas chamber920 is interposed between the pump cells 919 and 939. A reference gaschamber 940 is provided between the sensor cell 939 and the shieldingplate 94. Each of the solid electrolytic substrate layers 91 and 93 andthe shielding plate 94 is made of zirconia. The insulating spacer 92 ismade of alumina.

[0006] The pump cell 919 consists of the solid electrolytic substratelayer 91 and a pair of porous electrodes 911 and 912 provided onopposite surfaces of the solid electrolytic substrate layer 91. Thesensor cell 939 consists of the solid electrolytic substrate layer 93and a pair of electrodes 931 and 932 provided on opposite surfaces ofthe solid electrolytic substrate layer 91. A sample gas diffusive inletportion 921 introduces a sample gas to the sample gas chamber 920. Aprotector layer 900 is provided on an outer surface of the porouselectrode 911.

[0007] The pump cell 919 maintains the concentration of an oxygen gasresiding in the sample gas chamber 920 at a constant value by adjustingan oxygen gas amount introduced to or exhausted from the sample gaschamber 920. The sensor cell 939 detects an air-fuel ratio of the samplegas stored in the sample gas chamber 920.

[0008] More specifically, a comparator 950 compares a sensing signal ofthe sensor cell 939 with a reference voltage. A voltage responsive to anoutput of the comparator 950 is applied to the pump cell 919. The oxygengas amount varies in accordance with the applied voltage. Thus, anadjusted oxygen gas is introduced into or exhausted from the sample gaschamber 920. This realizes a feedback control of the concentration ofthe oxygen gas in the sample gas chamber 920. An obtained current duringthis feedback control is proportional to an air-fuel ratio of the samplegas. Thus, the air-fuel ratio is detectable from the measured currentvalue.

[0009] In general, the air-fuel ratio sensor functions properly onlywhen it has a high temperature exceeding a predetermined activetemperature. Hence, to assure an accurate operation, the multilayeredair-fuel ratio sensor 9 is equipped with a heater. The heater generatesa sufficient amount of heat to maintain the multilayered air-fuel ratiosensor 9 at a higher temperature exceeding its active temperature.

[0010] The ULEV law, effective from the year of 2,000 in CaliforniaState of the Unites States, forces the automotive makers to clear therequired levels of tough emission controls. To attain this goal, havingan excellent warmup ability is an essential factor to be realized forthe above-described multilayered air-fuel ratio sensor.

[0011] The planned target levels are significantly high. For example, anair-fuel ratio sensor must operate properly within a short period of 5seconds immediately after the engine is started up.

[0012] In this respect, the above-described conventional multilayeredair-fuel ratio sensor 9 has a drawback in that its heater is provided asa separate component. According to this arrangement, the heater mustincrease its temperature excessively to satisfy the rough regulations.The multilayered air-fuel ratio sensor is subjected to severe thermalshocks. It possibly causes cracks.

[0013] As one of practical methods for reducing the thermal shocks, itmay be possible to reduce an overall thickness of the multilayeredair-fuel ratio sensor. A heat capacity of the multilayered air-fuelratio sensor decreases in proportion to the reduction of its thickness.However, the mechanical strength of the multilayered air-fuel ratiosensor decreases correspondingly. This is not desirable.

[0014] The multilayered air-fuel ratio sensor usually receives variousexternal forces and vibrations, for example, when the multilayeredair-fuel ratio sensor is assembled with the heater or when themultilayered air-fuel ratio sensor is installed in an exhaust passage ofan internal combustion engine. Accordingly, any multilayered air-fuelratio sensor suffering from a decreased mechanical strength will bedamaged by such external forces and vibrations.

[0015]FIG. 15 shows a proposed arrangement for the above conventionalmultilayered air-fuel ratio sensor 9. A multilayered heater 99 isintegrated with the multilayered air-fuel ratio sensor 9 via aninsulating substrate layer 990. However, according to this arrangement,the size of the multilayered air-fuel ratio sensor 9 is substantiallyrestricted by the heat ability of the multilayered heater 99. Asdescribed above, increasing the heater temperature will cause theproblem that the multilayered air-fuel ratio sensor 9 is subjected tosevere thermal shocks. If the thickness of the multilayered air-fuelratio sensor 9 is reduced to solve this problem, the mechanical strengthwill be fatally deteriorated.

SUMMARY OF THE INVENTION

[0016] In view of the problems encountered in the prior art, an objectof the present invention is to provide a multilayered air-fuel ratiosensor having an excellent warmup ability and capable of effectivelypreventing the cracks from causing due to thermal shocks.

[0017] In order to accomplish this and other related objects, an aspectof the present invention provides a multilayered air-fuel ratio sensorcomprising a plurality of substrate layers comprising at least one solidelectrolytic substrate layer. At least one heterogeneous boundary layeris interposed between the plurality of substrate layers. Theheterogeneous boundary layer has a thickness in a range of 10 to 100 μm.The heterogeneous boundary layer absorbs thermal shocks or any otherstresses acting on the substrate layers and stops the growth of cracks.

[0018] Preferably, the heterogeneous boundary layer has a porous ratelarger than those of the neighboring substrate layers. The heterogeneousboundary layer has a sintering particle diameter larger than those ofthe neighboring substrate layers. The heterogeneous boundary layercomprises a component selected from the group consisting of alumina,spinel, and steatite. The heterogeneous boundary layer is interposedbetween a solid electrolytic substrate layer and an insulating substratelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription which is to be read in conjunction with the accompanyingdrawings, in which:

[0020]FIG. 1 is a perspective view showing an overall arrangement of amultilayered air-fuel ratio sensor in accordance with a first embodimentof the present invention;

[0021]FIG. 2 is a cross-sectional view showing the multilayered air-fuelratio sensor in accordance with the first embodiment of the presentinvention, taken along a line A-A of FIG. 1;

[0022]FIG. 3 is a cross-sectional view showing the multilayered air-fuelratio sensor in accordance with the first embodiment of the presentinvention, taken along a line B-B of FIG. 1;

[0023]FIG. 4 is a graph showing a relationship between the thickness ofa heterogeneous layer and the bending strength of the multilayeredair-fuel ratio sensor in accordance with the first embodiment of thepresent invention;

[0024]FIG. 5 is a graph showing a relationship between the presence ofthe heterogeneous layer and the bending strength of the multilayeredair-fuel ratio sensor in accordance with the first embodiment of thepresent invention;

[0025]FIG. 6 is a graph showing a relationship between the presence ofthe heterogeneous layer and the breaking strength of the multilayeredair-fuel ratio sensor in accordance with the first embodiment of thepresent invention;

[0026]FIG. 7 is a cross-sectional view showing an essential arrangementof a multilayered air-fuel ratio sensor having two insulating substratelayers in accordance with the first embodiment of the present invention;

[0027]FIG. 8 is a cross-sectional view showing an essential arrangementof a multilayered air-fuel ratio sensor having three insulatingsubstrate layers in accordance with the first embodiment of the presentinvention;

[0028]FIG. 9 is a cross-sectional view showing an essential arrangementof a multilayered air-fuel ratio sensor having three solid electrolyticsubstrate layers in accordance with a second embodiment of the presentinvention;

[0029]FIG. 10 is a cross-sectional view showing an essential arrangementof a multilayered air-fuel ratio sensor having one insulating substratelayer in accordance with the second embodiment of the present invention;

[0030]FIG. 11 is a cross-sectional view showing an essential arrangementof a multilayered air-fuel ratio sensor having two insulating substratelayers in accordance with the second embodiment of the presentinvention;

[0031]FIG. 12 is a cross-sectional view showing an essential arrangementof a multilayered air-fuel ratio sensor having two heterogeneous layersin accordance with a third embodiment of the present invention;

[0032]FIG. 13 is a perspective view showing an overall arrangement of aconventional multilayered air-fuel ratio sensor;

[0033]FIG. 14 is a cross-sectional view showing the conventionalmultilayered air-fuel ratio sensor shown in FIG. 13; and

[0034]FIG. 15 is a perspective view showing an improved arrangement ofthe conventional multilayered air-fuel ratio sensor shown in FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Preferred embodiments of the present invention will be explainedhereinafter with reference to attached drawings. Identical parts aredenoted by the same reference numerals throughout the views.

First Embodiment

[0036]FIGS. 1 through 8 are views showing preferable arrangements of amultilayered air-fuel ratio sensor 1 in accordance with a firstembodiment of the present invention. In the following explanation, anup-and-down direction is defined based on the layout shown in FIG. 1.Needless to say, an actual up-and-down direction of the multilayeredair-fuel ratio sensor 1 may be changed when it is installed on aninternal combustion engine.

[0037] The multilayered air-fuel ratio sensor 1 comprises a total offive solid electrolytic substrate layers 11 to 15, stacking from thebottom to the top in FIG. 1, each having the oxygen ion conductivity. Atotal of four heterogeneous layers 10 are alternately combined with thefive solid electrolytic substrate layers 11 to 15. These heterogeneouslayers 10 serve as boundary layers respectively interposed between twoneighboring solid electrolytic substrate layers. Each heterogeneouslayer 10 is 50 μm thick. Each of the solid electrolytic substrate layers11 to 15 is 240 μm thick. A total thickness of the multilayered air-fuelratio sensor 1 is 1.4 mm.

[0038] The solid electrolytic substrate layers 11 to 15 are made ofyttria partially-stabilized zirconia with an average sintering particlediameter of 2 to 3 μm. Each heterogeneous layer 10 is made of a materialselected from the group of alumina, spinel, and steatite. In thisembodiment, the heterogeneous layers 10 are made of α-alumina with anaverage sintering particle diameter of 3 to 4 μm.

[0039] The multilayered air-fuel ratio sensor 1 comprises a pump celland a sensor cell, as well as a multilayered heater integrally providedin the multilayered air-fuel ratio sensor 1.

[0040] As shown in FIGS. 1 to 3, the solid electrolytic substrate layer11 serves as a pump cell substrate having opposite surfaces on whichpumping electrodes 111 and 112 are provided in a back-to-backrelationship. A pinhole 110, serving as a sample gas introducingpassage, extends across the solid electrolytic substrate layer 11 from acenter of the upper (i.e., outer) pumping electrode 111 to a center ofthe lower (i.e., inner) pumping electrode 112.

[0041] The solid electrolytic substrate layer 12 has an aperturedefining a sample gas chamber 120. A lower surface of the solidelectrolytic substrate layer 11 defines a ceiling of the sample gaschamber 120. The lower pumping electrode 112 extends entirely along theceiling of the sample gas chamber 120. The upper pumping electrode 111extends along the upper surface of the solid electrolytic substratelayer 11.

[0042] An upper surface of the solid electrolytic substrate layer 13defines a bottom of the sample gas chamber 120. The pinhole 110communicated with the sample gas chamber 120.

[0043] The solid electrolytic substrate layer 13 serves as a sensor cellsubstrate having opposite surfaces on which sensing electrodes 131 and132 are provided in a back-to-back relationship. The upper sensingelectrode 131 extends along the bottom of the sample gas chamber 120.

[0044] The solid electrolytic substrate layer 14 has a slit defining areference gas chamber 140. A lower surface of the solid electrolyticsubstrate layer 13 defines a ceiling of the reference gas chamber 140.The lower sensing electrode 132 extends entirely along the ceiling ofthe reference gas chamber 140. A bottom of the reference gas chamber 140is defined by the heterogeneous layer 10 provided on an upper surface ofthe solid electrolytic substrate layer 15.

[0045] The solid electrolytic substrate layer 15 serves as a heatersubstrate. A heater element 150 is provided on the solid electrolyticsubstrate layer 15 via an insulation paste. The heater element 150 has apredetermined pattern extending along an upper surface of the solidelectrolytic substrate layer 15.

[0046] In FIG. 3, reference numerals 117, 118, 137 and 138 denote leadsconnecting respective electrodes to corresponding output terminals. FIG.1 shows an output terminal 119 connected via the lead 118 to the upperpumping electrode 111.

[0047] Next, a manufacturing procedure of the multilayered air-fuelratio sensor 1 will be explained.

[0048] First, a manufacturing method of a zirconic green sheet isexplained. The zirconic green sheet is used to form the solidelectrolytic substrate layers 11 to 15. A main material of the zirconicgreen sheet is a yttria partially-stabilized zirconia with an averageparticle diameter of 0.5 μm. This yttria partially-stabilized zirconiacomprises 6 mol % yttria and 94 mol % zirconia. The weighing capacity ofthe yttria partially-stabilized zirconia is 100 weight parts. Assubsidiary materials of the zirconic green sheet, an α-alumina is oneweight part, a PVB (polyvinyl butyral) is five weight parts, a DBP(di-butyl phthalate) is 10 weight parts, an ethanol is 10 weight parts,and a toluene is 10 weight parts.

[0049] The prepared yttria partially-stabilized zirconia, α-alumina,PVB, DBP, ethanol and toluene are mixed in a ball mill to obtain aslurry of them. The obtained slurry is configured into a plane sheetbody by using a doctor blade method. The fabricated sheet body is 0.3 mmthick in a dried condition. A total of five rectangular sheet bodies,each being 5 mm×70 mm, are cut out of this sheet body for the solidelectrolytic substrate layers 11 to 15.

[0050] Next, an electrically conductive Pt paste is screen printed inthe predetermined pattern on opposite surfaces of a first rectangularsheet body. This constructs the solid electrolytic substrate layer(i.e., sensor cell substrate) with the sensing electrodes 131 and 132.

[0051] In the same manner, the electrically conductive Pt paste isscreen printed on opposite surfaces of a second rectangular sheet body.This constructs the solid electrolytic substrate layer (i.e., pump cellsubstrate) 11 with the pumping electrodes 111 and 112. The pinhole 11 isopened across the solid electrolytic substrate layer 11. The diameter ofthe opened pinhole 11 is 0.5 mm.

[0052] Furthermore, the leads and the output terminals are provided atthe predetermined portions on the first and second rectangular sheetbodies of the sensor cell substrate 13 and the pump cell substrate 11.

[0053] Furthermore, an alumina insulating paste is printed on a thirdrectangular sheet body. Thereafter, an electrically conductive pastecontaining 90 wt % Pt and 10 wt % aluminum is printed on this thirdrectangular sheet body. This constructs the solid electrolytic substratelayer (i.e., heater substrate) 15 with the heater element 150. Theresistance value of the formed heater element 150 is 2.0 Ω at 20° C.

[0054] Furthermore, a fourth rectangular sheet body is provided with anaperture at a predetermined position. This constructs the solidelectrolytic substrate layer 12 defining the sample gas chamber 120. Afifth rectangular sheet body is provided with a slit at a predeterminedposition. This constructs the solid electrolytic substrate layer 14defining the reference gas chamber 140.

[0055] Next, a manufacturing method of an alumina green sheet will beexplained. The alumina green sheet is used to form the heterogeneouslayers 10. A main material of the aluminum green sheet is an α-aluminawith an average particle diameter of 0.3 μm. The weighing capacity ofthis α-alumina is 100 weight parts. As subsidiary materials (i.e.,binders), an acrylic resin is 30 weight parts and a toluene is 30 weightparts.

[0056] The prepared α-alumina, acrylic resin and toluene are kneaded bya roll mill to get a predetermined viscosity and then sheeted by a pressroller. The fabricated sheet body is 100 μm thick. A total of fourrectangular sheet bodies, each being 5 mm×70 mm, are cut out of thisplane sheet body for the heterogeneous layers 10. The rectangular sheetbodies are configured into the predetermined shape corresponding to theabove-described pinhole 110 and the gas chambers 120 and 140.

[0057] Subsequently, the rectangular sheet bodies of the solidelectrolytic substrate layers 11 to 15 are stacked or laminated in thepredetermined order with the alternately intervening rectangular sheetbodies of the heterogeneous layers 10, as shown in FIGS. 1 to 3. Then,the formed multilayered assembly is sintered at an atmosphericenvironment of 1,500° C. for one hour. Finishing this sinteringoperation obtains the multilayered air-fuel ratio sensor of thisembodiment.

[0058] Next, the performance of the multilayered air-fuel ratio sensorof this embodiment will be explained.

[0059] To check the performance, the multilayered air-fuel ratio sensorof the first embodiment was compared with several test samples. In theconducted comparative performance test, a plurality of green sheetsrespectively 0.35, 0.33, 0.25 and 0.21 mm thick were prepared for thesolid electrolytic substrate. Similarly, a plurality of green sheetsrespectively 0 (i.e., none), 40, 200 and 280 μm thick were prepared forthe heterogeneous layers. By appropriately assembling the prepared greensheets, various test samples of the multilayered air-fuel ratio sensorwere obtained. Each test sample was sintered. Each obtained air-fuelratio sensor was approximately 1.4 mm thick after finishing thesintering operation. This thickness is substantially the same as that ofthe multilayered air-fuel ratio sensor of the above-describedembodiment.

[0060] The thickness dispersion of each test sample was suppressedwithin 50 μm. The thickness of each heterogeneous layer was measuredthrough a SEM observation on a broken surface. The measured thicknessesof the heterogeneous layers were 0, 20, 50, 100 and 140 μm afterfinishing the sintering operation. In each of the five kinds of testsamples, a 3-point bending strength was measured in compliance withJISB0601. FIGS. 4 and 5 show the measured result.

[0061] According to the measured result of FIG. 4, the 3-point bendingstrength is larger than 250 MPa when the thickness of the heterogeneouslayer is in a range of 10 μm to 100 μm. The mechanical strength ispractically sufficient when the 3-point bending strength exceeds 250Mpa.

[0062] According to the measured result of FIG. 5, the tested air-fuelsample with a 10 μm heterogeneous layer shows an increased 3-pointbending strength that is approximately 1.4 times as large as that of thetested air-fuel sample having no heterogeneous layer.

[0063] As shown in FIG. 4, the 3-point bending strength is maximized inthe vicinity of 50 μm. In other words, the optimum thickness of theheterogeneous layer resides near 50 μm.

[0064]FIG. 6 shows a result of a spalling strength test. This shows arelationship between the presence of the heterogeneous layer and thebreaking strength of the multilayered air-fuel ratio sensor. To measurethe breaking strength, the test samples were held in a dried environmentof a predetermined temperature for 30 minutes and then soaked in water.

[0065] As apparent from the graph of FIG. 6, a large thermal shockdurability (temperature difference) is obtained by providing theheterogeneous layer in the multilayered air-fuel ratio sensor. It isthus confirmed that the breaking strength can be improved by providingthe heterogeneous layer. The air-fuel ratio sensor is installed in anexhaust pipe of an internal combustion engine. The air-fuel ratio sensorin an engine startup condition is usually subjected to condensed waterremaining in the exhaust pipe. However, in such a severe condition, thepresent invention can effectively prevent the multilayered air-fuelratio sensor from causing a thermal stress cracking by the provision ofthe heterogeneous layer.

[0066] Next, functions and effects of the above-described embodiment ofthe present invention will be explained.

[0067] According to the first embodiment, the multilayered air-fuelratio sensor 1 has heterogeneous layers 10 each serving as a boundarylayer interposed between two neighboring solid electrolytic substratelayers. When a thermal stress or any other stress acts on the solidelectrolytic substrate layers 11˜15, a small crack may appear. However,the heterogeneous layer acts as a buffer for absorbing the stresses. Thegrowth of the crack is surely prevented by the heterogeneous layer.Thus, the multilayered air-fuel ratio sensor 1 is free from the fatalcracking.

[0068] Thus, the first embodiment of the present invention can provide amultilayered air-fuel ratio sensor robust against thermal shocks. Thisallows an increased heater temperature. Needless to say, increasing theheater temperature is effective to improve the warmup ability.Accordingly, the first embodiment of the present invention provides amultilayered air-fuel ratio sensor having an excellent warmup ability.

[0069] According to the first embodiment, the solid electrolyticsubstrate layers 11˜15 are made of yttria partially-stabilized zirconia.The heterogeneous layer 10 is made of alumina. The thermal expansioncoefficient of the yttria partially-stabilized zirconia is substantiallythe same as that of the alumina. No damage occurs due to a thermalexpansion coefficient difference between the yttria partially-stabilizedzirconia and the alumina.

[0070] As described above, the first embodiment of the present inventionprovides the multilayered air-fuel ratio sensor excellent in the warmupability and robust against the thermal shocks.

[0071]FIG. 7 shows a modified arrangement of the multilayered air-fuelratio sensor of the first embodiment wherein two solid electrolyticsubstrate layers 14 and 15 are replaced by insulating substrate layers24 and 25. More specifically, the multilayered air-fuel ratio sensor 1shown in FIG. 7 comprises three solid electrolytic substrate layers11˜13 and two insulating substrate layers 24 and 25 stacked to form amultilayered construction. The insulating substrate layers 24 and 25 aremade of alumina. A total of four heterogeneous layers 10 are alternatelycombined with the five substrate layers 11˜13 and 24˜25. Theseheterogeneous layers 10 serve as boundary layers respectively interposedbetween two neighboring substrate layers for absorbing the stresses.

[0072]FIG. 8 shows another modified arrangement of the multilayeredair-fuel ratio sensor of the first embodiment wherein three solidelectrolytic substrate layers 12, 14 and 15 are replaced by insulatingsubstrate layers 22, 24 and 25.

[0073] Both of the modified arrangements shown in FIGS. 7 and 8 bringsubstantially the same functions and effects as those of theabove-described FIGS. 1˜3 embodiment.

[0074] As apparent from the foregoing description, the present inventionprovides the multilayered air-fuel ratio sensor comprising a pluralityof substrate layers comprising at least one solid electrolytic substratelayer. At least one heterogeneous layer is interposed between two of theplurality of substrate layers. The heterogeneous layer serves as aboundary layer that absorbs thermal shocks or any other stresses actingon the substrate layers and stops the growth of cracks.

[0075] The heterogeneous layer has a thickness in a range of 10 to 100μm. When the thickness of the heterogeneous layer is smaller than 10 μm,the effects of the present invention may not be obtained. When thethickness of the heterogeneous layer is larger than 100 μm, theheterogeneous layer may behave as a bulk body that is weak againstthermal shocks. The cracks may be generated. Furthermore, the thicknessof the sensor increases. This deteriorates the warmup ability.

[0076] Preferably, the heterogeneous layers are provided at all ofboundaries of the substrate layers. However, the effect of the presentinvention can be obtained by providing at least one heterogeneous layer.

[0077] Preferably, the heterogeneous layer has a porous rate larger thanthose of the neighboring substrate layers. When the porous rate islarge, the buffer effect of the heterogeneous layer can be enhanced.

[0078] Preferably, the heterogeneous layer has a sintering particlediameter larger than those of the neighboring substrate layers. When thesintering particle diameter is large, the buffer effect of theheterogeneous layer can be enhanced.

[0079] Preferably, the heterogeneous layer comprises a componentselected from the group consisting of alumina, spinel, and steatite.These materials are insulating materials capable of serving as aninsulating substrate layer. The thermal expansion coefficients of thesematerials are substantially the same as that of the solid electrolyticsubstrate layer. No damage occurs due to a thermal expansion coefficientdifference between the heterogeneous layer and the slid electrolyticsubstrate layer.

[0080] When the solid electrolytic substrate layer is made of a zirconicmaterial, it is preferable to use a heterogeneous layer made of aluminain view of the insulation ability and the thermal expansion coefficient.A sintering operation produces a thermal expansion coefficientdifference due to a thermal hysteresis. The combination of the zirconicmaterial and the alumina material is preferable to suppress a stresscaused by such a thermal expansion coefficient difference.

[0081] Preferably, the heterogeneous layer is interposed between a solidelectrolytic substrate layer and an insulating substrate layer.

[0082] Preferably, the multilayered air-fuel ratio sensor comprises amultilayered heater.

Second Embodiment

[0083]FIGS. 9 through 11 are views showing preferable arrangements of amultilayered air-fuel ratio sensor 3 in accordance with a secondembodiment of the present invention. The multilayered air-fuel ratiosensor 3 comprises a total of three substrate layers.

[0084] According to an arrangement shown in FIG. 9, the multilayeredair-fuel ratio sensor 3 comprises an upper solid electrolytic substratelayer 31 with upper and lower electrodes 311 and 312 provided on opposedsurfaces thereof in a back-and-back relationship. A medium solidelectrolytic substrate layer 32 is provided with a slit defining areference gas chamber 320. A lower surface of the upper solidelectrolytic substrate layer 31 defines a ceiling of the reference gaschamber 320. The lower electrode 312 extends entirely along the ceilingof the reference gas chamber 320.

[0085] A bottom of the reference gas chamber 320 is defined by an uppersurface of a heterogeneous layer 10 mounted on a lower solidelectrolytic substrate layer 33. The lower solid electrolytic substratelayer 33 serves as a heater substrate on an upper surface of which aheater element 330 is provided via an insulating paste layer. Anotherheterogeneous layer 10 is interposed between the upper and medium solidelectrolytic substrate layers 31 and 32.

[0086]FIG. 10 shows another arrangement of the multilayered air-fuelratio sensor 3 of the second embodiment wherein the lower solidelectrolytic substrate layers 33 is replaced by an insulating substratelayer 43.

[0087]FIG. 11 shows another arrangement of the multilayered air-fuelratio sensor 3 of the second embodiment wherein both of the medium andlower solid electrolytic substrate layers 32 and 33 are replaced byinsulating substrate layer 42 and 43.

[0088] Both of the modified arrangements shown in FIGS. 10 and 11 bringsubstantially the same functions and effects as those of theabove-described FIG. 9 embodiment.

Third Embodiment

[0089]FIG. 12 is a view showing a preferable arrangement of amultilayered air-fuel ratio sensor 5 in accordance with a thirdembodiment. The multilayered air-fuel ratio sensor 5 comprises threesolid electrolytic substrate layers 11˜13 and two insulating substratelayers 24˜25. One heterogeneous layer 101 is interposed between thesolid electrolytic substrate layers 11 and 12. Another heterogeneouslayer 102 is interposed between the solid electrolytic substrate layer13 and the insulating substrate layer 24. Both of the heterogeneouslayers 101 and 102 are insulating layers made of alumina.

[0090] According to the arrangement of the third embodiment, theheterogeneous layer 101 has a function of insulating the solidelectrolytic substrate layer 11 from the solid electrolytic substratelayer 12 or vice versa as well as a function of absorbing the thermalshocks.

[0091] The FIG. 12 embodiment brings substantially the same functionsand effects as those of the above-described first embodiment.

[0092] This invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof. The presentembodiments as described are therefore intended to be only illustrativeand not restrictive, since the scope of the invention is defined by theappended claims rather than by the description preceding them. Allchanges that fall within the metes and bounds of the claims, orequivalents of such metes and bounds, are therefore intended to beembraced by the claims.

What is claimed is:
 1. A multilayered air-fuel ratio sensor comprising:a plurality of substrate layers comprising at least one solidelectrolytic substrate layer; and at least one heterogeneous boundarylayer interposed between said plurality of substrate layers, saidheterogeneous boundary layer having a thickness in a range of 10 to 100μm.
 2. The multilayered air-fuel ratio sensor in accordance with claim 1, wherein said heterogeneous boundary layer has a porous rate largerthan those of neighboring substrate layers.
 3. The multilayered air-fuelratio sensor in accordance with claim 1 , wherein said heterogeneousboundary layer has a sintering particle diameter larger than those ofneighboring substrate layers.
 4. The multilayered air-fuel ratio sensorin accordance with claim 1 , wherein said heterogeneous boundary layercomprises a component selected from the group consisting of alumina,spinel, and steatite.
 5. The multilayered air-fuel ratio sensor inaccordance with claim 1 , wherein said heterogeneous boundary layer isinterposed between a solid electrolytic substrate and an insulatingsubstrate.